Evidence for L-glutamate as the neurotransmitter of the squid giant synapse

AMPA/kainate and NMDA-like glutamate receptors at the chromatophore neuromuscular junction of the squid: role in synaptic transmission and skin patterning

Glutamate receptor types were examined at the chromatophore synapses of the squids Alloteuthis subulata and Loligo vulgaris, where nerve-induced muscle contraction causes chromatophore expansion. Immunoblotting with antibody raised against a squid AMPA receptor (sGluR) demonstrated that AMPA/kainate receptors are present in squid skin. Application of l-glutamate evoked chromatophore muscle contractions in both ventral and dorsal skins, while NMDA was only active on a subpopulation of dorsal chromatophores. In dorsal skin, neurotransmission was partly blocked by either AMPA/kainate receptor antagonists (CNQX and DNQX) or NMDA receptor antagonists (AP-5 and MK-801) or completely blocked by simultaneous application of both classes of antagonists. In isolated muscle fibres, ionophoretic application of l-glutamate evoked fast inward CNQX- and DNQX-sensitive currents with reversal potentials around +14 mV and a high conductance to Na+. In fibres from dorsal skin only, a slower outward glutamate-sensitive current appeared at positive holding potentials. At negative potentials, currents were potentiated by glycine or by removing external Mg2+ and were blocked by AP-5 and MK-801. Glutamate caused a fast, followed by a slow, transient increase in cytoplasmic Ca2+. The slow component was increased in amplitude and duration by glycine or by lowering external Mg2+ and decreased by AP-5 and MK-801. In cells from ventral skin, no 'NMDA-like responses' were detected. Thus, while AMPA/kainate receptors mediated fast excitatory synaptic transmission and rapid colour change over the whole skin, activation of both AMPA/kainate and NMDA-like receptors in a subpopulation of dorsal chromatophores prolonged the postsynaptically evoked Ca2+ elevation causing temporally extended colour displays with behavioural significance.

The effect of L-glutamate on the afferent resting activity in the cephalopod statocyst

The effects of bath application of L-glutamate and of excitatory amino acid agonists and antagonists on the resting activity of afferent crista fibers were studied in isolated preparations of the statocyst of the cuttlefish, Sepia officinalis. L-Glutamate (threshold 10(-5) M) and its agonists quisqualate and kainate (thresholds 10(-6) M) increased the resting activity in a dose-dependent manner. Glutamine (threshold 10(-5) M) was also excitatory, while D-glutamate had no effect. Also, no obvious excitatory effects were seen for NMDA and L-aspartate, nor was any antagonistic effect seen for the selective NMDA-receptor antagonist D-2-amino-5-phosphonovaleric acid (D-AP-5). The spider toxin Argiotoxin636 (threshold 10(-11) M), 2-amino-4-phosphonobutyric acid (AP-4), glutamic acid diethyl ester (GDEE), gamma-D-glutamylaminomethyl-sulfonic acid (GAMS), and kynurenic acid decreased the resting activity and effectively blocked or reversed the effect of L-glutamate and its non-NMDA agonists. Preliminary experiments with statocysts from the squid Sepioteuthis lessoniana and the octopod Octopus bimaculoides gave comparable results. All data show that in cephalopod statocysts L-glutamate, via non-NMDA receptors, has an excitatory effect on the activity of afferent fibers, an effect consistent with its possible function as a hair cell transmitter.

Spider toxin and pertussis toxin differentiate post- and presynaptic glutamate receptors

A specific blocker of the postsynaptic glutamate receptors was found in the venom of the spider Nephila clavata. The toxin (JSTX) preferentially blocks quisqualate-type glutamate receptors in the crustacean neuromuscular synapse, squid giant synapse and hippocampal neurons in slice preparations. Following determination of the structure of JSTXs, a main component JSTX-3 with its analogs was chemically synthesized and used for the study of structure-activity relationships. 125I-labeled JSTX-3 and biotinylated JSTX-3 were synthesized for histochemical and biochemical studies of the glutamate receptors. The labeled JSTXs enabled visualization of the glutamate receptors in lobster muscle, rat cerebellum and hippocampus. By use of JSTX and pertussis toxin, a novel type of glutamate receptor (GluB receptor) was found in the crustacean neuromuscular synapse. While the postsynaptic glutamate receptor was blocked by JSTX, GluB receptor was insensitive to JSTX, but it was blocked by pertussis toxin, indicating involvement of inhibitory GTP-binding protein. Injection of GTP gamma S in the presynaptic axon mimicked the presynaptic glutamate potentials and caused presynaptic inhibitory action. Thus two types of biological toxins clearly separate the pre- and postsynaptic glutamate receptors.

Effects of a spider toxin (JSTX) on hippocampal CA1 neurons in vitro

The effect of a toxin (JSTX) obtained from Nephila clavata (Joro spider) on the CA1 pyramidal neurons of the hippocampus was studied using slice preparations. JSTX blocked the excitatory postsynaptic potentials (EPSPs) in the pyramidal neuron evoked by Schaffer collateral stimulation but was without effect on the antidromic action potentials or on the resting conductance. Depolarization induced by ionophoretic application of glutamate was readily suppressed by JSTX but aspartate-induced depolarization was much less sensitive to the toxin. Among preferential agonists activating 3 receptor subtypes for excitatory amino acids, quisqualate responses were most effectively suppressed by JSTX. Kainate responses were similarly suppressed but in some cells higher concentration of the toxin was needed to block the responses. N-methyl-D-aspartate (NMDA) responses were the least sensitive to JSTX but they were suppressed by +/- 2-amino-5-phosphonovaleric acid (APV). Long term potentiation (LTP) once it had taken place was not completely inhibited by APV. In the presence of JSTX, however, LTP was blocked and tetanic stimuli produced only a short-lived potentiation. In Mg2+ free solution, an orthodromic stimulation evoked repetitive spike responses which were superimposed on the depolarization following the initial spike. APV suppressed the depolarization and associated spikes leaving an orthodromic response which was sensitive to JSTX. The results suggest that JSTX blocks EPSPs in CA1 pyramidal neurons which are mediated by non-NMDA type receptors.

Evidence that glutamate mediates axon-to-Schwann cell signaling in the squid

High-frequency stimulation (100 Hz) of isolated giant axons of the small squid Alloteuthis subulata and the large squid Loligo forbesi caused the periaxonal Schwann cell resting potential (Em = -40 mV) to hyperpolarize up to 11 mV in direct proportion to train duration and action potential amplitude. In both species, the Schwann cell also hyperpolarized up to 17 mV with the application of L-glutamate (10(-9) to 10(-6) M), in a dose-dependent manner. By contrast, in the presence of 10(-8) M d-tubocurarine (d-TC) to block the cholinergic component of the Schwann cell response, Schwann cells depolarized 8-9 mV during electrical stimulation of the axon or application of L-glutamate. In the presence of 10(-5) M 2-amino-4-phosphonobutyrate (2-APB), the hyperpolarization to glutamate and to axon stimulation was blocked, whereas the cholinergic (carbachol-induced) hyperpolarization was unaffected. In experiments with Alloteuthis, L-aspartate (10(-7) M) also caused a Schwann cell hyperpolarization, but this was not blocked by 2-APB. In tests with glutamate receptor agonists and antagonists, quisqualate (10(-5) M) produced a hyperpolarization blocked by 10(-4) M L-glutamic acid diethylester (GDEE), which also blocked the response to axonal stimulation. Kainic acid (10(-4) M) also caused a hyperpolarization, but n-methyl-D-aspartate (NMDA; 10(-4) M), ibotenate (10(-5) M), alpha-amino-3-hydroxy-5-methyl-isoxazole proprionate (AMPA; (10(-4) M), and isethionate (10(-5) M) had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

An autoradiographic study of binding of iodinated spider toxin to lobster muscle

Distribution of the binding sites of Joro spider toxin (JSTX), a specific inhibitor of the glutamate receptors in the crustacean neuromuscular synapse, was studied by using autoradiography. JSTX was synthesized and made radioactive by conjugation with iodine-125. 125I-JSTX irreversibly blocked the excitatory postsynaptic potentials of the lobster neuromuscular synapse in a similar manner as the natural spider toxin. Light microscopic autoradiography of 125I-JSTX treated muscle showed sporadic aggregates of reduced silver grains on the surface of muscles. Electron microscopy of adjoining ultrathin sections revealed that these spots corresponded to the fraction of sarcolemma apposed to axonal terminals with or without synaptic junctional profiles. This finding gives morphological support to the formulation that JSTX binds to the glutamate receptor-ion channel molecules.